1. Field of the Invention
The present invention relates to a battery charger and, more particularly, to a battery charger for rapidly and safely charging lithium ion batteries which automatically compensates for anticipated changes of the resistance of the battery charging circuit (i.e. external battery terminals, wires, and internal battery cell resistance) over time, due to, for example, oxidation of the external battery contacts, and provides a maximum and constant current to a battery cell over the anticipated resistance range of battery charging circuit in order to minimize the charging time of the battery cell.
2. Description of the Prior Art
Battery chargers for charging lithium-ion-type batteries are known in the art. Examples of such battery chargers are disclosed in U.S. Pat. Nos. 5,670,862; 6,002,237 and 6,859,014. Such lithium ion battery chargers are also disclosed in U.S. Patent Application Publication Nos. U.S. 2001/0011882 A1; U.S. 2003/0057920 A1 and U.S. 2003/0141850 A1; as well as Japanese Patent No. JP 20-00277151 and Chinese Patent No. CN 1269616. As is known in the art, such lithium ion batteries require constant current (CC) and constant voltage (CV) charging. In particular, initially such lithium ion batteries are charged with a constant current. In the constant current mode, the charging voltage is typically set to a maximum level recommended by the LiIon cell manufacturer based on safety considerations, typically 4.2V per cell. The charging current is limited by the circuit to a design level, based on the cell capability, charge time, needs and cost. Once the battery cell voltage rises sufficiently, the voltage drop across the series resistances in the battery charging circuit forces the charging current to drop below an initial charge current level. In particular, when the battery cell voltage Vb approaches the charging voltage Vc, the charging current tapers according to the formula: I=(Vc-Vb)/Rs, where I=the charging current, Vc=the charging voltage, Vb=the battery cell voltage and Rs=the resistance of the charging circuit including the external contact resistance, the resistance of the battery terminals and wires used to connect the circuit, as well as the internal resistance of the battery cell. As such, during the last portion of the charging cycle, typically about the last ⅓, the battery cell is charged at a reduced charging current, which means it takes more time to fully charge the battery cell.
In order to decrease the time to fully charge such lithium ion battery cells, many known battery chargers take into account the voltage drop across the battery circuit elements in order to maximize the amount of charging current during a constant current mode. One method of determining the voltage drop of the battery circuit resistance elements is to take the difference between the closed-circuit voltage (i.e. charging voltage), and the open circuit voltage, which is the battery cell voltage with no charging current. The closed-circuit voltage represents the voltage of the battery cell plus the voltage drops in the circuit as a result of resistance in the battery circuit, such as the battery terminals and the internal resistance of the battery cell. By subtracting the closed-circuit voltage from the open-circuit voltage, the voltage drop across the battery resistance circuit elements can be determined.
Various known battery chargers use this voltage drop to drive the battery charging voltage during a constant current mode in order to increase the amount of current applied to the battery cell during a constant current mode. By increasing the amount of current applied to the battery cell during a constant current mode, the battery cell is charged much faster.
Examples of lithium ion battery charger circuits that compensate for the voltage drop in the battery circuit in order to increase the charging current and thus decrease the charging time for a lithium ion battery are disclosed in U.S. Pat. Nos. 5,670,862; 6,859,014 and 6,002,237. More particularly, the '862 Patent teaches a compensation circuit for compensating for predetermined voltage drop in the battery circuit. The compensation circuit includes an operational amplifier as well as a resistor sized to take into account the expected electrical series resistance of the battery circuit. The compensation circuit is based on an assumed initial voltage drop across the various resistance elements in the circuit and compensates for this voltage drop to maintain a predetermined charging current during a constant current charging mode. Unfortunately, the resistance of the various resistance elements change over time due to various factors including oxidation of the external battery contacts used to connect the battery cell to the battery charger. The compensation circuit disclosed in the '862 patent does not take into account such changes in resistance over time. Accordingly, in time, the charging time of the battery cell increases.
The charging technique disclosed in the '014 patent also takes into account the voltage across various battery circuit elements. In particular, the voltage drop across the battery circuit elements is monitored. During a normal charging condition, a “full” charging current is applied to the battery. If the monitored voltage drop exceeds a predetermined value, the charging current to the battery is reduced to a low level for a predetermined time. After the predetermined time period has elapsed, the “full” charging current is restored to the battery. Unfortunately when the battery is being charged at a reduced current level, the amount of time required to fully charge the battery is increased.
U.S. Pat. No. 6,002,237, assigned to the same assignee as the assignee of the present invention, discloses a rapid charging method for charging a lithium ion battery cell that also takes into account the voltage drop across the external battery contacts s as well as the other battery circuit elements. The battery charging circuit disclosed in the '237 patent monitors the voltage drop across the various circuit elements in the battery circuit, as well as the battery cell voltage, to make sure that the maximum cell voltage is not exceeded. The battery cell voltage is measured by periodically interrupting the charging current flow to the battery cell and taking a voltage measurement. The potential difference between the battery cell voltage (i.e. open circuit voltage) and the battery circuit elements (i.e. closed-circuit voltage) is periodically determined. This potential difference represents the voltage drop across various elements in the battery circuit. This potential difference is then used to adjust the charging voltage to the battery during a constant current mode. By adjusting the charging voltage during a constant current mode, the charging current increases, thus decreasing the time for charging the lithium ion battery.
The system disclosed in the '237 patent requires periodic measurement of the potential difference between the battery cell voltage and the closed-circuit voltage, which includes the battery cell voltage as well as the potential drop across the battery circuit elements. Although the system disclosed in the '237 Patent provides a rapid charge methodology for lithium ion batteries, the intent of the system disclosed in the '237 patent is to reduce the charging time by increasing the charging current once the battery cell voltage starts to increase and then reduce the charging current if no compensation for the increasing battery cell voltage Vb takes place. Unfortunately, the battery charging circuit disclosed in the '237 patent does not take into account the changes in the series resistance values in the circuit over time and thus the effectiveness of the circuit to rapidly charge a lithium ion battery wanes over time as the resistance of the series resistance elements increases.
There is a need to further reduce the charging time of such lithium ion batteries.